The present application relates to a system for modulating light being guided in absence of any imaging optics through consecutive diffraction processes by diffraction surfaces spaced from one another at relatively large distances.
System of such art are used in interferometers to analyze preferably in conjunction with photoelectric detector means the movement of an object carrier of such an interferometer from the one-dimensional or multi-dimensional light modulations proportional to distance or path, to angle or to speed of the object carrier.
The state of the art of interferometers may be ascertained by reference to U.S. Pat. No. 3,591,841, the disclosure of which is incorporated herein.
The state of the art of using optical diffraction systems useful in interferometers may be ascertained by U.S. Pat. No. 3,904,295, the disclosure of which is incorporated herein.
The state of the art of optical systems for analyzing the movement of a movable grating and evaluating the signals therefrom in a push-pull amplifier circuit are disclosed in U.S. Pat. Nos. 2,942,119; 3,198,061; 3,482,107; 3,525,569 and 3,591,841, and the Journal of the Optical Society of America, Volume 53, No. 12, pages 1420 and 1421 (December 1963), the disclosures of which are incorporated herein.
The state of the art of apparatus for generating phase modulated electrical signals in response to a measured angular or linear displacement may be ascertained by reference to U.S. Pat. Nos. 3,768,022; 3,781,119; and 3,833,299, the disclosures of which are incorporated herein.
The disclosure of U.S. Pat. No. 3,320,852 is incorporated herein to further explain the combination of objective 15 of the present invention as shown in FIG. 1, with optical diffraction grating 13 and the projection of images onto plane 14. This patent also shows the utility of the apparatus of the present invention for producing a plurality of images of a single object.
For purposes of modulation, it is known from the art to pass light through at least three consecutive diffraction processes mentioned above in the optical path of light. The first of which starts from one point of the first one of at least three grating structures utilized to diffract light and effects the splitting of the light into at least two diffraction orders impinging on the second one of such structures having a grating constant that ascertains the condensation of the diffracted light into one point on the third grating structure.
Such light being guided always into the same direction of diffraction is fulfilling the coherence condition. The rays or beams of light describing the diffracted light waves encompass a polygon and the spacing between the first and the second grating structure and between the last and the penultimate one as well as the grating constants between the first and the last one of such grating structures are equal. It is stated that no imaging takes place between diffractions.
Multiple consecutive diffractions are known from instruments described among others by Gerasimow & Rassudova in:
Optics and Spectroscopy 14, pages 215-219 PA1 Optics and Spectroscopy 14, pages 296-297 PA1 Optics and Spectroscopy 19, pages 152-156 PA1 Applied Optics, Vol. 6, No. 11 (Nov. 1967) pages 1861-1865 PA1 Optical Technology, Vol. 38, No. 10 (Oct. 1971) pages 588-590
and further by reference to the publication of: Weinberg in the Journal of Sci. Inst., Vol. 36, (May 1959) at page 228; the disclosures of which are incorporated herein.
Having in mind the conditions for spacing and diffraction as mentioned above, these grating structures are useful for various cases of transmission and reflection.
These systems of the art described operate in principle upon the fact that relative movement between the gratings in a direction transverse to the rulings results in a change in the direction of light output from the gratings. The directional change can be visually observed as a transverse shift in moire fringe position and when appropriate optics are employed in the light entering or emerging such systems the change in light output directions can be translated into a generally cyclic fluctuation in the intensity of light transmitted. Photoelectrical means are commonly used to convert the varying light intensity to a comparably varying electrical signal which is employed to trigger electronic counting means in order to indicate the amount of relative movement between the grating elements.
Such systems provide light modulations in various directions, and depending on the diffraction orders of these modulations, are converted into electrical push-pull signals by means of a receiver detector system. However, (2k + 1).multidot..pi./2 phase shifted signals are lacking in the systems of the known art to generate rotational fields required for detecting in a simple manner the relative directions of motion.